Molecular model of the opioid painkiller buprenorphine. This drug is used to treat moderate pain and to help with withdrawal from opioid addiction. Buprenorphine is a partial opioid antagonist, that is it acts on some of the same receptors as opioid drugs such as heroin, but not on others. This means that it can reduce the side effects of withdrawal without the causing the high or dangerous respiratory suppression of other opioids. Atoms are represented as spheres and are colour-coded: carbon (light blue), hydrogen (white), oxygen (red) and nitrogen (dark blue).